A process for dyeing textile materials is known to usually include a step of checking the color fastness thereof. Such inspection should be performed before unloading the materials from the machines in which the dyeing is carried out, in order to subject the materials, in case of unfavourable outcome, to a washing or cleaning treatment. During this step, in fact, it may be economically more advantageous to reprocess the materials than unloading them and classifying or selling them as second-rate or defective products.